In nuclear reaction, particularly as regards the fission (or splitting) process, a neutron bombards a nucleus of a heavy material and produces two or more fission fragments and additional free neutrons. The reactor-fuel material is typically an isotope of uranium, such as uranium 235. The reaction continues when a released free neutron in turn strikes additional quantities of the fissile material; and in this regard a chain reaction is started and sustained. The reactor fuel can take the form of a fluid, such as an aqueous solution of enriched uranium; but in the main, the fuel is solid, either metallic uranium or a ceramic such as uranium oxide. The solid fuel material is fabricated into various small plates, pellets, pins, etc. which are usually clustered together in an assemblage called a fuel element. Almost all solid fuel elements are cladded with a protective coating or sheath that prevents direct contact between the fuel material and the reactor coolant. The cladding also serves as part of the structure of the fuel element. A zirconium alloy is commonly used as the cladding material in power reactors; whereas aluminum might more typically be used in research reactors.
The reaction of the fuel elements generates heat, and this is dissipated typically by means of a coolant passed through the reactor. In many instances, specific coolant paths are defined, such as through specific passages within which the fuel elements are housed, and the coolant is forced by appropriate pump means over these fuel elements. The coolaant can be water operating as either liquid or steam at pressures upward of 2000 psi, or the coolant can be a liquid metal such as sodium or a sodium-potassium mixture. The coolant systems further can vary, where only a single primary coolant like water is used and this is directed via a closed primary loop through a power-generating turbine or the like; or the primary coolant may be directed via a closed loop to a heat exchanger, and a second coolant is used in a second closed loop including this heat exchanger and the power-generating equipment. The particular system for generating and utilizing the heat is of no concern to this invention, but this background has been given merely for a more basic understanding of the operation of the system.
The primary coolant, as noted, passes in proximate contact over the cladded-fuel elements; and sound cladding isolates or separates the coolant from the radioactive material. However, in the event of any breach in the cladding, the coolant directly contacts the fuel. The radioactive discharge can then in turn be conveyed via the coolant throughout the entire coolant system to contaminate the system.
As noted, the reaction releases radioactive elements having varying half-life decay rates. These radioactive elements may include, for example, krypton, xenon, zirconium and barium, each of which has a specific radioactivity and a specific half-life. Should the radioactive elements admix with the coolant, the increase in the specific radiation level of the coolant can be detected by means of a GELI detector (a germanium and lithium gamma-ray detector) and/or by means of a gas analysis device.
Also given off as part of a radioactive discharge when the reaction occurs are at least nine different isotopes that not only give off the typical gamma rays of radioactivity, but also give off what is known as delayed neutrons. These isotopes, or delayed-neutron emitters, would include bromine, iodine, and tellurium to name just a few. Each of these delayed-neutron emitters is soluble in liquid sodium (the coolant) so that it readily blends in with the coolant, should a fuel element cladding breach occur, and flows with the coolant throughout the system. This also allows detection means for the delayed neutrons to be set up adjacent the coolant system at a location remote from the reactor core.
Each delayed-neutron emitter has a determinable and specific half-life decay rate. This will vary from a half-life as short as two seconds up to a half-life as much as 54 seconds. The half-life begins its countdown at the birth of the delayed-neutron emitter which is simultaneously with the nuclear reaction or the splitting of the particular atom in question.
This introduces also the concept of the parent-daughter, and the occurrence of a precursor and the release of the delayed neutron. In this regard, the reactions are sequential in nature and provide that a precursor occurs originally and decays to a new isotope, now called the daughter. The time sequence of this is all determined by specific known natural laws of decay.
The delayed-neutron detector is a gas-filled instrument, typically a boron fluoride (BF.sub.3) or a helium isotope (He.sup.3). The release of delayed neutrons from delayed-neutron emitters in the coolant passing near the detector ionizes the gas within the detector and results in a pulse count that increases with increased numbers of delayed-neutron emitters near the detector. The number of pulses per unit time determines the magnitude of the delayed-neutron signal.
The magnitude of the delayed-neutron signal is not only a function of what materials are involved, but also is a function of the size of the breach. In other words, a small breach or opening in the cladding would release a small amount of delayed-neutron emitters as compared to a larger breach at the same location. Also, the temperature of the fuel elements influences the rate of release of the delayed-neutron emitters where a hot element forces a greater rate of release than does a cooler element.
The physical location in the fuel element itself of the birthplace of the delayed-neutron emitter, and the holdup time required for the delayed-neutron emitter to even reach the breach in the cladding vary and have to be considered. The proximity of the breach to the detector can create additional uncertainty as to the detected count, because the travel time required for the infiltrated coolant to move to the detector can be short in one circumstance and can be quite long in another circumstance. The half-life decay would, of course, decrease the total count when a longer holdup time and/or travel time is involved. Also, the transit time can increase or decrease because the coolant flow may not be uniform or may vary as between one flow passage and another.
In this regard, the typical reactor has several fuel element passages each having its proportioned amount of coolant, where all coolant flow is combined at common inlet and/or discharge manifolds. The delayed-neutron detector can commonly be located in a coolant loop downstream of the reactor. The count of released delayed neutrons, however, could heretofore only be used in a very broad sense to determine when a breach in the cladding has occured, but the same could not accurately determine the severity or specific location of the breach.